The most common method of joining a variety of substrates such as glass, rubber, leather, paper, porcelain, wood and metals is by application of an adhesive substance (often referred to as cement, glue or paste) to the respective adherends. Depending on the particular adhesive utilized, the glue or cement is applied to one or both of the substrates which are thereafter either immediately joined or, in the case of soluble adhesives, the solvent is allowed to evaporate until the adhesive has reached its desired tacky stage prior to joining of the substrates. In any event, the glue or cement has already reached, or is about to reach, its desired degree of tackiness--i.e. is an active adhesive composition--within a short time after application thereof so that the adherends must be set and joined with speed and precision immediately after the adhesive has been applied. This means that active adhesives have only a limited application life, also known as pot life. Moreover, the curing of the active adhesive, once exposed to air or mixed with the catalyst, proceeds to full cure without the possibility of being controlled in the sense of retarding, discontinuing or otherwise manipulating the curing process or re-setting the adherends. Once applied, conventional glues are also usually difficult to remove.
Some of the above-mentioned disadvantages have been partially overcome by incorporating minute, liquid-containing capsules into a perforated sheet material, the entrapped or enveloped liquid being released upon the application of pressure. Thus, U.S. Pat. No. 3,472,675 discloses a self-wetting sheet material which has a pattern of closely spaced perforations filled with a dried polymeric material in which microencapsulated droplets of solvents are incorporated. Also disclosed is a perforated sheet material in which each perforation is filled with a microencapsulated epoxy resin and catalyst. The sheet is dry to the touch and the two components are brought into coacting relationship by rupturing the microcapsules. However, once the microcapsules are ruptured, the components unite and are expressed as an active adhesive which must be utilized immediately since the curing reaction has been initiated.
A particular advantage of the adhesive composition of the present invention is that it may be applied with the same equipment used for surface coating--such as roller, flow, curtain and knife coaters--and that the point in time when the composition becomes an active adhesive by the application of external pressure, abrasive motion, shear stress or other mechanical manipulation which causes the microcapsules to rupture, thereby releasing the catalyst, can essentially be controlled by the user. Due to the substantially unlimited pot life of the instant adhesive composition, the joining of two adherends need not be rushed but may be conducted with more accuracy and precision.
One of the objects of the present invention is therefore to provide a curable adhesive composition which is applied in liquid or viscous form as, for example, by brushing or rolling upon a substrate, and which will not attain its activity (i.e. cure or harden) before the substrates are united as desired and after sufficient pressure is applied thereto.
Another object of the present invention is to provide an adhesive composition which, if mistakenly applied to a surface, can be easily removed therefrom by simply wiping off the yet inactive composition.
Microencapsulation is a known technique which therefore does not form a part of this invention separate and distinct from the composition as claimed. Microencapsulation is exemplified by the disclosure of U.S. Pat. Nos. 3,324,500; 3,472,675; 3,598,123; 3,640,629 and the above-mentioned reference. In addition, U.S. Pat. No. 3,686,701 discloses a cosmetic applicator containing rupturable microencapsules which contain both solvents for the removal of nail enamel and perfume to mask the odor of the solvents. And U.S. Pat. No. 3,691,270 discloses a cosmetic makeup removing composition incorporated in a flexible support throughout which are distributed microcapsules containing the cosmetic makeup removing or treating composition.